Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 1 XRD RAPID SCREENING SYSTEM FOR COMBINATORIAL CHEMISTRY Bob B. He, John Anzelmo, Peter LaPuma, Uwe Preckwinkel, Kingsley Smith Bruker Analytical X-ray Systems Madison, Wisconsin, USA ABSTRACT Combinatorial chemistry refers to techniques to fabricate, test, and store the resulting data for a material library containing tens, hundreds or even thousands different materials or compounds. Combinatorial investigations require rapid screening techniques to test and evaluate variations of composition, structure and property within a material library. X-ray diffraction is one of the most suitable screening techniques because abundant information can be revealed from the diffraction pattern and X-ray diffraction is fast and non-destructive. A two-dimensional X-ray diffraction system designed for rapid screening, D8 Discover GADDS for Combinatorial Screening, is introduced by Bruker AXS. INTRODUCTION The concept of combinatorial chemistry was introduced about 30 years ago [1]. Instead of the traditional way of making and testing a few new materials one at a time, the combinatorial technology allows scientists to fabricate, test, evaluate and store the resulting data for a material library containing tens, hundreds or even thousands of different materials or compounds. Combinatorial chemistry has become increasingly accepted by academia, government and industry in the past few years. Excellent results have been achieved in discovery and syntheses of new phosphors, catalysts, zeolites and new drugs [2-8]. Combinatorial chemistry requires rapid screening techniques to test and evaluate the variation of composition, structure and property of the entire material library. X-ray diffraction is one of the most suitable rapid screening techniques because of the penetrating power of the x-ray beam, it is nondestructive to samples, data collection is rapid, and there is a lot of useful information about the materials contained in the diffraction pattern. X-ray diffraction, especially two-dimensional x-ray diffraction, can be used to measure the structural information of a material library with high speed and high accuracy [9,10]. An XRD system introduced by Bruker AXS D8 Discover GADDS for Combinatorial Chemistry is designed for the rapid screening of combinatorial libraries. The system design is based on two-dimensional x-ray diffraction (XRD 2 ) theory. A two-dimensional multiwire area detector can collect a large area of a diffraction pattern with high speed, high sensitivity, low noise, and in a real time mode. A 2D-diffraction pattern contains information about the structure, quantitative phase contents, crystal orientation and deformation. The laser/video system ensures that each sample is aligned accurately on the instrument center. The x-ray beam is collimated to various sizes from 1000 to 50 µm. The vertical theta-theta geometry and horizontally mounted XYZ stage allow one to load the combinatorial library with ease, even for loose powders or liquids. The GADDS software helps one to select and save a record of the screening area and steps. The diffraction results are processed and mapped to the screening grid based on the userselected parameters.
Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 2 TWO-DIMENSIONAL X-RAY DIFFRACTION (XRD 2 ) A two-dimensional X-ray diffraction (XRD 2 ) system has both the capability of acquiring diffraction patterns in 2D space simultaneously, and analyzing the 2D diffraction data accordingly. An XRD 2 system consists of at least one 2D detector, x-ray source and optics, sample positioning stage, sample alignment and monitoring device as well as corresponding computer control and data reduction and analysis software. The diffraction pattern from a polycrystalline (powder) sample consists of a series of diffraction cones if a large number of crystals (oriented randomly in the space) are irradiated by the incident x-ray beam. Figure 1 is a schematic of how x-ray diffraction occurs from a polycrystalline sample. For simplicity, it shows only two diffraction cones, one for forward diffraction and one for backward diffraction. The diffraction measurement in the conventional diffractometer is confined within a plane, here referred to as diffractometer plane. A point detector makes a 2θ scan along a detection circle. If a one-dimensional position sensitive detector (PSD) is used in the diffractometer, it will be mounted on the detection circle. Since the variation of the diffraction pattern in the direction (Z) perpendicular to the diffractometer plane is not considered in the conventional diffractometer, the corresponding structure information will be either ignored, or has to be measured by various additional sample rotations. Figure 1. Diffraction patterns in 3D space from a powder sample and the diffractometer plane of a conventional θ-2θ diffractometer. When one uses a 2D detector, diffraction data is no longer limited to measurement only in the diffractometer plane. Instead, the whole, or a large portion of the diffraction rings can be measured simultaneously, depending on the detector size and position. Since the variations of diffraction intensity in all directions are equally important, a point focus beam is normally used for XRD 2. THE XRD 2 SYSTEM FOR COMBINATORIAL SCREENING Figure 2 shows a schematic and photo of an XRD 2 combinatorial screening system. All components are mounted on a vertical θ-θ goniometer. X-ray tube and optics are mounted on a dovetail track, referred to as θ 1 track. A 2D detector is mounted on a dovetail track - θ 2 track.
Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 3 The XYZ stage is located with X-Y in the horizontal surface and Z vertical. A laser/video system is used to align and monitor the sample. Figure 2. Schematic (top) and photo (bottom) of an XRD 2 combinatorial screening system; including a 2D detector, X-ray generator, X-ray optics (monochromator and collimator), theta-theta goniometer, XYZ sample stage, and a laser/video sample alignment and monitoring system. The X-ray beam is monochromatized with either a graphite monochromator or a multi-layer mirror. The X-ray beam can be collimated to various sizes by using a pinhole collimator or monocapillary. The multiwire detector has a pixel resolution of 100 µm or 200 µm with frame size of 1024x1024 or 512x512. The detector can be set at a sample-to-detector distance between 6 cm to 30 cm depending on the application: For larger angular coverage at short distance (65
Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 4 measuring range at 6cm) or high angular resolution at long distance (0.02 resolution at 30 cm). The XYZ stage has travel range of 100 mm x 100 (or 150) mm x 100 mm, and a maximum loading capacity of 10 kg with a 12.5 µm position accuracy and a 5 µm repeatability. The laser beam and optical axis of the microscope intercept at the instrument center so the sample position can be determined by the laser spot position on the sample image as is shown in Figure 3. Since the video image can be captured with the safety enclosure closed, the video microscope can record the image and position of each cell of the material library during the data collection. Figure 3. The principle of the laser/video function. The laser beam and optical axis of the zoom video cross at the instrument center. DIFFRACTION MAPPING AND RESULTS DISPLAY Figure 4. 2D frames with integrated diffraction profiles, each from a single library point.
Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 5 The multiwire area detector can capture a large area of diffraction data containing information for various applications, such as: Phase ID (qualitative or quantitative); Percent Crystallinity; Particle Size and Shape; Texture; and Stress. Figure 4 shows two examples of the diffraction frame and integrated diffraction profile, each from a single library point. Almost all the parameters measured by X-ray diffraction can be used for the screening of material libraries. The data collection grid, including XYZ coordinates of all the cells, is determined by GADDS software based on the coordinates of the two cells at extreme positions (low-left and upper-right) and step size between cells. The data collection is automatically scanned over all the cells in the material library. Selection of screening parameters includes integrated intensity, maximum intensity, peak width (FWHM), peak 2θ position, crystallinity (% internal, % external and % full) and various stress components. The screening results can be displayed in color coded map, 3D surface plot, or pass/fail map with user defined criteria as is shown in Figure 5. Figure 5. The screening parameters are displayed in color scale, 3D surface plot or pass/fail plot on the material library map. SUMMARY X-ray diffraction is one of the preferred methods for high-throughput combinatorial screening. A two-dimensional X-ray diffraction (XRD 2 ) system designed for combinatorial chemistry contains equipment with the latest technological advances needed to perform high throughput, easy-to-use screening on a wide variety of analytical parameters. REFERENCES [1] Joseph J. Hanak, J. Mater. Sci., 5, 964, 1970. [2] Mark S. Lesney, Today s Chemist at Work, January, 1999, 37. [3] Rob Brown, Today s Chemist at Work, January, 1999, 49. [4] John S. MacNeil, Today s Chemist at Work, December, 1999, 22. [5] Stu Borman, Chemical & Engineering News, March 8, 1999, 33. [6] Ron Dagani, Chemical & Engineering News, March 8, 1999, 51. [7] X. D. Xiang, et al, Science 1995, 268, 1738-1740. [8] B. Jandeleit, et al, Angew. Chem., Int. Ed. Engl. 1999, 38, 2494. [9] B. B. He, U. Preckwinkel and K. L. Smith, Advances in X-ray Analysis, Vol. 43, 1999. [10] Jens Lein, et al, Angew. Chem., Int. Ed. Engl. 1998, 37, 3369.